1. Field of the Invention
The invention relates to modified polyol esters of rosin and more particularly relates to Diels-Alder adducts of esters of rosin.
2. Brief Description of the Prior Art
It is well known that gum or wood rosin may be readily reacted with alpha, beta-unsaturated carboxylic acids to obtain the corresponding Diels-Alder adducts; see for example U.S. Pat. Nos. 3,291,785 and 3,562,243.
It is also well known that tall oil rosins may be esterified by reaction with polyols; see for example U.S. Pat. No. 3,780,012.
Other U.S. patents representative of descriptions of modified rosins are the U.S. Pat. Nos. 2,532,120; 2,563,871; 2,973,332; and 3,106,550.
Little is known about resins made by first preparing a rosin polyol ester of low residual acid number and then reacting this ester with a dienophile under Diel-Alder reaction conditions to obtain a simple adduct of the ester. One resin is this class is the highly modified ethylene glycol ester reported in Polish Pat. No. 124,426. This material is not claimed to be a pure Diels-Alder adduct. It contains about 2-4 moles of maleic anhydride per mole of rosin diene (reactive abietic-types), added in the presence of phosphoric acid catalyst.
It is clear from these prior art descriptions that the usual method of preparing "maleic-modified rosin esters" is to simply heat a mixture of maleic anhydride, polyol, and rosin until the desired acid number is achieved. Because considerable reaction between the polyol and the maleic carboxylic acid groups results in extensive esterification prior to esterification of the relatively unreactive rosin carboxylic acid group, this method affords a resin of high molecular weight and very broad molecular weight distribution. This type of resin is not always desired. The process of the present invention is an improvement in the preparation of rosin resins of low molecular weight and narrow molecular weight distribution containing intact the residual functionality of the dienophile used to modify the rosin ester.